1. Field of the Invention
The present invention relates to an energy storage device including plural capacitor cells.
2. Description of the Related Art
An image forming apparatus, such as a multifunction printer (MFP), a printer, or a facsimile machine (FAX) is configured to form an image on an image forming medium, such as normal paper or an overhead projector (OHP) sheet, using one of various image forming methods.
Among the various image forming methods that may be used by an image forming apparatus, the electrophotographic method is widely used by current image forming apparatuses owing to its advantages with regard to speed, quality, and costs.
The electrophotographic method includes a process of forming a toner image on an image forming medium and fixing the toner image onto the image forming medium using heat and pressure. It is noted that the heat roller method is widely used for performing such a process owing to its advantages with regard to speed and safety.
The heat roller method involves pressing a pressure roller against a heat roller that is heated by a heat generating member such as a halogen heater to form a so-called nip portion where the rollers are pressed against each other, and passing an image forming medium holding a pre-fixed toner image between the rollers to fix the toner image onto the image forming medium.
The heat roller is a metal roller that is primarily made of metal such as steel or aluminum so that it has a relatively large heat capacity. Thus, the heat roller has a drawback in that it requires a relatively long time of around a few minutes to ten plus a few minutes to be heated to a usage temperature of approximately 180° C. at which the heat roller may be used.
Accordingly, in a typical image forming apparatus, even during standby mode when an image forming process such as printing is not performed by a user, electrical power is supplied to the heat roller to maintain the temperature of the heat roller at a preheated temperature that is slightly lower than the usage temperature of the heat roller. In this way, for example, when a print execution request is issued by a user, the heat roller may be heated to the usage temperature in a relatively short period of time so that printing may be performed without a long waiting time from the time the print execution request is issued to the time printing operations are started.
However, because the heat roller is maintained at a preheated temperature slightly lower than the usage temperature, even when the image forming apparatus is not used (i.e., in standby mode), excessive power that is not directly necessary for image formation is consumed by the image forming apparatus.
In view of the growing awareness of issues related to environmental protection, specifications and guidelines related to energy conservation of office automation (OA) equipment are being established as is exemplified by the Energy Star program, which is being promoted by the Japanese Ministry of Economy, Trade and Industry and the U.S. Environmental Protection Agency (EPA).
Considering such a background, current image forming apparatuses are desirably configured to conform to such energy conservation specifications and guidelines. It is noted that reducing the above-described excessive power consumption that is not directly necessary for image formation is one effective way of satisfying the requirements of such specifications and guidelines. To reduce such excessive power consumption, power supply to a fixing device of an image forming apparatus during standby mode is desirably set to zero.
However, when the power supplied to the fixing device is set to zero in a conventional image forming apparatus, a relatively long time is required for the heat roller to reach its corresponding usage temperature upon being restarted so that the waiting time from the time a print execution request is issued to the time printing operations are started may be relatively long. Such an effect can cause stress on the user. Accordingly, an image forming apparatus conforming to a given energy conservation program is generally required to implement measures for enabling its heat roller to be quickly heated to its corresponding usage temperature upon being restarted. For example, the Zero Energy Star Mode (ZESM) requires its qualified energy-conserving image forming apparatus to be able to restart operations from standby mode within ten seconds.
It is noted that increasing the input energy per time unit, namely the rated apparent power, of the image forming apparatus is one way of reducing the time required for heating the heat roller to its corresponding usage temperature. For example, there are image forming apparatuses with high speed printing capabilities that are adapted to use a power supply of 200 V.
However, the power supply in a conventional office environment is typically 100 V/15 A so that the maximum electrical power of the power supply is 1500 W. In order to adapt an office environment to provide a power supply of 200 V, special engineering has to be performed on the present power supply environment which cannot be considered a very practical solution. In another example, an image forming apparatus has been developed that uses two lines of the power supply of 100 V/15 A in order to increase the total input power. However, the image forming apparatus having such a configuration may only be used under an environment where two lines of power outlets are located close to each other.
Thus, more practical measures are in demand for increasing the maximum input energy (power supply) of an image forming apparatus to raise the temperature of the heat roller to its corresponding usage temperature in a short period of time.
In view of such a demand, a method for enabling energy conservation by increasing the maximum power supply of an image forming apparatus has been proposed that involves using a rechargeable auxiliary power supply.
It is noted that a lead storage battery and a nickel-cadmium storage battery are representative examples of rechargeable auxiliary power supply.
However, such a battery (i.e., secondary battery) is prone to degradation upon being repeatedly recharged and its charge capacity may decrease as a result, and the life of such a battery becomes shorter as the discharge current of the battery is increased. For example, even in the case of using a nickel-cadmium storage battery, which is generally known to last for a relatively long time even when being charged/discharge at a large charge/discharge current, the number of times the nickel-cadmium storage battery may be recharged before breaking down is approximately 500-1000 times so that if the battery is recharged 20 times a day, the life of the battery may reach its end in approximately one month. Thus, using such a battery may not be a practical solution owing the burden of having to frequently exchange the battery and high running costs from using many batteries, for example. Further, it is noted that several hours may be required for fully charging such a large capacity battery so that it is not suited for use in an application that requires charging/discharging to be performed many times per day, for example.
In view of such a problem, Japanese Patent No. 3588006 (Publication No. 2000-315567) discloses a technique that involves using a large capacity capacitor such as an electric double layer capacitor (simply referred to as ‘capacitor’ hereinafter) as an auxiliary power supply in place of a secondary battery.
By using an energy storage device made up of a capacitor as an auxiliary power supply, the time required for a fixing device to reach its corresponding usage temperature (i.e., rise time) may be a relatively short time of a few seconds to ten plus a few seconds, and power exceeding the maximum power that may be provided by a commercial power supply may be supplied to the fixing device. Thus, a reliable and durable fixing device with a short rise time may be provided.
However, owing to the general characteristic that a capacitor has a relatively small energy density, an energy storage device that uses a capacitor including that disclosed in the above-mentioned Japanese Patent No. 3588006 may need a large number of capacitors in order to set the maximum power that may be supplied by the energy storage device to an adequately high level in consideration of cases in which a large amount of power has to be supplied such as the case of supplying power to an image forming apparatus. As a result, the volume (size) of the energy storage device may be enlarged, for example.
It is noted that power supply from an energy storage device to an image forming apparatus may vary over time depending on factors such as the operation status of the image forming apparatus.
For example, during a period of about ten seconds corresponding to the rise time for starting the image forming apparatus, close to 2000 W of power may have to be supplied to the image forming apparatus from the energy storage device. On the other hand, when successive printing is performed by the image forming apparatus after the rise time, the power that needs to be supplied from the energy storage device may be reduced to approximately 500 W. Further, the power supply from the energy storage device to the image forming apparatus may also vary during the successive printing operations. Specifically, during the successive printing operations performed right after the rise time, the fixing device may still not be adequately heated so that a power of approximately 500 W may be required. However, after around ten plus a few seconds, the required power may be reduced further to approximately 200 W.
It is noted that the maximum output density P of a capacitor is inversely proportional to the internal resistance R of the capacitor. Accordingly, in a case where a large amount of power has to be extracted from a capacitor such as during the rise time of the image forming apparatus as is described above, the internal resistance of the capacitor is desirably low. That is, in order to extract a large amount of power from an energy storage device, capacitors with low internal resistance are preferably used in the energy storage device.
(Large Capacity Capacitor)
It is noted that the internal resistance of a capacitor correlates with the capacity of the capacitor; namely, a capacitor with a large capacity tends to have low internal resistance whereas a capacitor with a small capacity tends to have high internal resistance.
The following is a list of methods for reducing the internal resistance of a capacitor module having plural capacitors in cell units (referred to as ‘capacitor cells’ hereinafter) that are serially connected.
(1) Use capacitor cells with capacities that are larger than necessary.
(2) Connect plural small capacity capacitor cells in parallel.
However, in the case of implementing the above-described methods (1) or (2), the size of the energy storage device including the capacitor cells may be enlarged.
(High Output Capacitor Supplying a Large Amount of Power)
It is noted that a high output capacitor supplying a large amount of power has a small normalized internal resistance value (i.e., internal resistance value obtained by normalizing an RC value (time constant), which is the product of the capacitance C and the internal resistance R of the capacitor cell) and is capable of outputting a large amount of power.
However, such a high output capacitor is larger in size compared to a capacitor outputting standard power so that an energy storage device using such a high output capacitor may become larger as well.
As can be appreciated from the above descriptions, in the case where capacitor cells with low internal resistance are used, the size of the energy storage device inevitably becomes large. In this case, even when an apparatus such as an image forming apparatus only requires a large amount of power during one portion of its overall power supply time, an energy storage device of the apparatus still has to be relatively large to supply such large amount of power.